1. Field of the Invention
The present invention relates to an electric potential sensor, and more particularly to an electric potential sensor to detect, in a non-contact manner, the charge on a photosensitive drum of an electrophotographic device such as a photocopy machine.
2. Description of the Related Art
The construction of a conventional oscillation-capacity type electric potential sensor is illustrated in FIG. 3.
An electric potential sensor 100 includes a piezoelectric tuning fork 11 which acts as a mechanical oscillator. The piezoelectric tuning fork 11 includes a vibrating body 11a of tuning fork type formed of a metal such as elinvar, a driving piezoelectric element 11b for driving the vibrating body lia is provided a first arm of the vibrating body 11a, and a feedback piezoelectric element 11c for feeding back the signal to the driving piezoelectric element 11b is provided on a second arm of the vibrating body 11a. A detection electrode 12 is formed on the surface of the second arm of the vibrating body 11a, i.e., on the surface of the second arm of the piezoelectric tuning fork 11, and the piezoelectric tuning fork 11 is arranged so that the detection electrode 12 is arranged opposite to a object 13 to be measured. In FIG. 3, the detection electrode 12 is shown separate from the piezoelectric tuning fork 11 to aid in easy understanding of the figure.
The driving piezoelectric element 11b is connected to an output end of a self-oscillation circuit 15, and the feedback piezoelectric element 11c is connected to an input end of the self-oscillation circuit 15. The detection electrode 12 is connected to a signal-processing circuit 20 comprising an impedance converter 21, an AC amplifier 22, a synchronous detection and smoothing circuit 23, and a DC amplifier 24.
The operation of the electric potential sensor 100 is as follows.
The drive signal is output from an output end of the self-oscillation circuit 15. The drive signal is applied to the driving piezoelectric element 11b to cause the driving piezoelectric element 11b to be distorted and a first arm of the vibrating body 11a to be vibrated.
Due to the vibration of the first arm and the tuning fork shape of the vibrating body 11a, a vibration having an opposite phase to that of first arm is generated in the second arm of the vibrating body 11a. The vibration of the second arm of the vibrating body 11a causes the feedback piezoelectric element 11c to be distorted. A feedback signal is therefore generated from the feedback piezoelectric element 11c and the piezoelectric tuning fork 11 is self-oscillated by the application of the feedback signal to the input end of the self-oscillation circuit 15.
The object 13 to be measured is charged to the electric potential V.sub.HV, and an electric field E is generated between the object 13 measured and the detection electrode 12. When the piezoelectric tuning fork 11 is caused to vibrate, the distance between the detection electrode 12 formed on the piezoelectric tuning fork 11 and the object 13 fluctuates periodically and the electrostatic capacitance generated between the detection electrode 12 and the object 13 is changed periodically. This induces a charge at the detection electrode 12, thereby generating an AC signal. Because the AC signal is proportional to the electric potential V.sub.HV of the object 13, the detection output signal corresponding to the electric potential V.sub.HV of the object 13 can be obtained by applying the AC signal in the signal-processing circuit 20.
The above-mentioned conventional electric potential sensor has a problem that it is difficult to correctly measure the charged electric potential of the object to be measured. This is because the amplitude of the vibration of the piezoelectric tuning fork is changed due to the temperature characteristic of the piezoelectric tuning fork, the vibration leakage from a support member to support the piezoelectric tuning fork, etc., with the result that the output signal of the electric potential sensor is not stable.
An electric potential sensor of oscillation-capacity type designed to solve this problem is disclosed in Japanese Unexamined Patent Publication No. 60-29673. The electric potential sensor detects the amplitude of the piezoelectric tuning fork using a photosensor and maintains the amplitude of the piezoelectric tuning fork to be constant accordingly. However, since the electric potential sensor requires a photosensor, a new problem is raised that the wiring in the vicinity of the piezoelectric tuning fork becomes complicated.
Another electric potential sensor intended to solve the above-mentioned problem includes an electric potential sensor of chopper type disclosed in Japanese Examined Utility Model Publication No. 5-2865. This electric potential sensor eliminates the signal attributable to the amplitude of the piezoelectric tuning fork from the output electric potential by dividing the output voltage of the electric potential sensor and the output voltage of the chopper part.
However, the sensitivity of the detection signal is degraded because the output voltage is divided. Further, because the output voltage of the electric potential sensor and the output voltage of the chopper part both include noise, the noise components are not eliminated even through the division and, indeed, are further increased through the division. In addition, a new problem is raised that the S/N ratio of the detection signal is reduced.